1. Field of the Invention
The present invention relates to a method for controlling an electric injection unit and, more particularly, to an injection speed control procedure for timing the change of the injection speed of an injection molding machine which carries out an injection process having a plurality of different injection stages to inject a molten resin into the cavity of a mold.
2. Description of the Related Art
FIG. 5 is a diagram of assistance in explaining a conventional injection molding method, in which the position S of an injection screw with respect to a reference position or time from a reference time point is measured on the horizontal axis, and the injection speed V of the injection screw or injection pressure P is measured on the vertical axis. The injection process represented by FIG. 5 comprises an injection phase S1 for injecting a molten resin into a cavity formed in a mold by advancing the injection screw and changing the injection speed V of the injection screw stepwise at a plurality of stages, and a dwelling phase S2 for holding the injection pressure for molding after the completion of the injection phase by changing the injection pressure P stepwise at a plurality of stages.
Generally, in the injection phase S1, a pressure control valve included in a hydraulic circuit is set for a high pressure, and the opening of a flow control valve is varied according to the position of the injection screw with respect to a reference position or time elapsed from the start of an injection molding process so that the speed of the injection screw changes stepwise at a plurality of stages with the distance of movement of the injection screw or time elapsed from the start of the injection molding process. When filling up the cavity with a molten resin, the flow control valve is regulated to control the speed of a piston fitted in an injection cylinder, i.e., the injection speed of the injection screw.
After the cavity has been filled up with the molten resin, the opening of the flow control valve is adjusted to a relatively small opening for the dwelling phase S2. In the dwelling phase S2, the opening of the pressure control valve is regulated to apply a predetermined pressure constantly to the molten resin filling up the cavity.
Generally, desired injection speed is changed stepwise at time points where the measured position of the injection screw coincides with predetermined injection speed changing points to control the injection speed in the injection phase S1. FIG. 6 is a diagram representing such an injection speed control procedure, in which desired injection speeds V1, V2, V3 and V4 are given at the start of the injection process and at speed changing points 1, 2 and 3, respectively.
When injecting the molten resin into the cavity, a screw tip, i.e., a check ring, for preventing the back-flow of the molten resin on the front side of the injection screw closes at different time points in different shot cycles.
The conventional injection control method sets beforehand changing points where injection speed and injection pressure are changed. Therefore, the actual strokes of the injection screw in each of the stages for the set injection speeds and the set injection pressures are reduced if the closing operation of the check ring is delayed.
An injection region SA representing an amount of work done in a shot cycle A in which the check ring closes at an early time point, which is indicated by a continuous line A in FIG. 6, in the injection process is greater than an injection region SB representing an amount of work done in a shot cycle B in which the check ring closes at a late time point, which is indicated by a broken line B in FIG. 6, in the injection process.
Thus, in a shot cycle in which the check ring closes at a late time point, the actual amount of work for injection is reduced even if the set injection time and set injection speed are not changed. Such an injection process affects the weight and the quality of the products; that is the variation of the time point when the check ring closes causes the variation of the weight and quality of the products.
A prior art control method previously proposed in JP-A No. 8-66944 determines a change time point when an injection phase is to be changed for a dwelling phase on the basis of a measured pressure of the hydraulic fluid or the molten resin. This prior art control method determines the change time point on the basis of only the pressure measured at one time point and does not take into consideration the amount of work done in the injection process. Accordingly, this prior art control method is unable to solve satisfactorily problems causing the variation of the weight and the quality of products.
Possible causes residing in an injection molding machine and causing molding faults due to such an unstable injection process include, in addition to the variation of the time point when the check ring closes, the change of the density of the metered resin, the variation of the metered volume and the variation of the injection speed.
Various control methods giving considerations to such problems have been proposed. An injection molding machine disclosed in JP-A No. 8-66944 carries out one of those prior art control methods. This injection molding machine is provided with a position sensor for measuring the position of an injection screw, a pressure sensor, and a speed calculating device for calculating the derivative of an injection stroke measured by the position sensor with respect to time or an injection screw speed measuring device. In this injection molding machine, the injection phase is divided into a plurality of stages and the injection screw is advanced at different injection speeds at different stages, respectively. In this injection molding machine, a reference pressure for deciding the closing operation of a check ring is set at an optional point in an injection phase. The difference between a position of the injection screw at a moment when the injection pressure reaches the reference pressure and a position of the same at a moment when the injection pressure reaches the reference pressure in a predetermined reference shot cycle is calculated and stored. The injection speed is changed when the injection screw reaches the reference injection speed changing points before the injection pressure reaches the reference pressure, and the injection speed is changed when the injection screw reaches injection speed changing points determined by correcting the initial injection speed changing points by using the difference after the injection pressure has reached the reference pressure.
This control method is able to suppress the adverse effect of the irregular closing characteristic of the check ring to the least extent. However, since the effects of the change of the density of the metered resin, the variation of the metered volume and the variation of the injection speed on the condition of the molten resin are not taken into consideration in devising this control method, this control method is not necessarily satisfactory in the effect of perfectly solving the problems attributable to the unstable injection process due to the different conditions of the molten resin in different shot cycles.
Another injection molding machine proposed for the same purpose in JP-B No. 2-39973 measures the hydraulic pressure applied to an injection ram and the injection speed of an injection screw, the product of hydraulic pressure and injection speed measured from the start of the injection process to the termination of the dwelling phase is integrated with respect to time, the integral thus obtained is compared by a comparator with values in a predetermined reference range set by a reference range setting device and, if the integral is outside the reference range, a signal is generated.
An injection pressure monitoring system proposed in JP-B No. 4-57490 to monitor a condition of an injection process on the basis of time taken by an injection molding machine to raise injection pressure comprises: a pressure sensor disposed in a part of the injection molding machine in which pressure is produced; a pressure comparing means which compares a pressure measured by the pressure sensor with a predetermined pressure and provides a pressure coincidence signal when the pressure measured by the pressure sensor coincides with the predetermined pressure, an integrator which starts an integrating operation to integrate a fixed reference voltage at a moment when a start signal is given thereto, provides an integral voltage corresponding to an integral obtained by the integrating operation, and stops the integrating operation upon the reception of the pressure coincidence signal from the pressure comparing means; a first comparing unit which compares the integral voltage with a predetermined upper limit voltage and provides an upper limit signal when the integral voltage coincides with the upper limit voltage; an upper limit alarm generating device which generates an upper limit alarm when the pressure coincidence signal is generated after the upper limit signal; a second comparing unit which compares the integral of voltage with a predetermined lower limit voltage and provides a lower limit signal when the integral voltage coincides with the lower limit voltage; and a lower limit alarm generating device which generates a lower limit alarm when the pressure coincidence signal is generated before the lower limit signal. This injection pressure monitoring system monitors the injection pressure of the injection molding machine, and generates an alarm when time taken by the injection molding machine to raise the injection pressure to the predetermined pressure is outside the predetermined reference range.
The foregoing two prior art techniques are designed to examine conditions and generate an alarm when the condition meets a specific condition and takes nothing into consideration about a method and an apparatus for the feedback control of an injection molding machine.
A prior art control method proposed in JP-B No. 3-1139 to stabilize the quality of moldings by always supplying a fixed quantity of energy into the cavity of a mold during an injection phase comprises the steps of measuring injection pressure and injection speed, integrating the product of injection pressure and injection speed with respect to time during an injection phase, comparing the integral with a predetermined reference value, and starting a dwelling phase upon the coincidence of the integral with the predetermined reference value.
However, this prior art control method is able to correct only a speed region immediately before the dwelling phase, and the amount of work done by the injection process is unable to follow properly the variation of the condition of the molten resin with time, the control method is unsatisfactory in achieving stable molding to produce good moldings.
Problems exactly the same as those unsolved problems in those prior art methods of controlling the speed of an injection molding machine are found in an electric injection machine in which an injection screw is driven for forward and backward movement by a servomotor to meter and inject a molten resin, and a mold is closed and opened by a servomotor.
To solve those problems, various methods of controlling the injection operations of electric injection molding machines have been proposed. A method of controlling an injection molding machine proposed in JP-B No. 7-119033 employs a servo driving unit provided with a servo amplifier for the feedback control of injection speed, and an arithmetic and control unit for controlling the general operations of the injection molding machine according to predetermined molding conditions. When an actual injection pressure pattern showing the variation of injection pressure in a primary injection process deviates from a predetermined average injection pressure pattern, the arithmetic and control unit corrects a predetermined injection speed pattern representing desired values for servo control so that actual injection pressure patterns in the next and the following injection processes may be within an allowable range.
This control method achieves a molding operation capable of producing good moldings by suppressing the variation of injection pressure in the primary injection process to the least possible extent and takes nothing about the amount of work into consideration. Consequently, the variation of the weight and the quality of produces cannot satisfactorily be reduced.
A method of controlling the injection operation of an electric injection molding machine is disclosed in JP-B No. 7-39123. An electric injection molding machine disclosed in JP-B No. 7-39123 is provided with a constant setting means for setting an optional constant included in an relational expression: (Actual filling pressure).times.(Actual injection speed)=Constant necessary for calculating a limit filling pressure and an actual injection speed to be used as thresholds for deciding whether or not injection speed control is to be removed, a speed control means for controlling injection speed so that the product of actual filling pressure measured by a pressure sensor and actual injection speed is equal to the constant set by the constant setting means, and a pressure control means for comparing an actual filling pressure measured by the pressure sensor in an injection phase and a limit injection pressure set by the constant setting means, and removing injection speed control when the actual filling pressure reaches the limit filling pressure to maintain pressure constant for filling control.
This control method reduces injection speed as actual filling pressure approaches the limit filling pressure to improve the dimensional accuracy of moldings and takes nothing about the amount of work into consideration. Consequently, the variation of the weight and the quality of produces cannot satisfactorily be reduced.